Implantable medical devices have been widely used for more than 40 years in various surgical applications. For example, in the fracture fixation operations, medical devices are used to address bone fractures by attaching a reinforcing rod or a plate or a cage to a fractured bone so that the broken ends may be stabilized to promote fusion and consequent healing. In the sports medicine area, medical devices are used to repair and augment soft tissues, such as anterior cruciate ligament (ACL) replacement. Implantable medical devices such as screws are used to affix autografts, allografts, xenografts, or bone fragments to bone structure of a patient.
Though metals have long been used as implantable medical devices, biodegradable materials, materials that degrade in the body, and then either absorb into, or are excreted from, the body, have been used in as alternatives to metals. Specifically designed biodegradable materials can have mechanical properties that begin to approach those of bone in some applications. As healing progresses, the stiffness and strength of the biodegradable material implant gradually decrease, transferring loads from the implant to the healing bone tissue.
In medical procedures, the easy movement of a surface of a device with respect to tissue is important in reducing damage to both the surface and to the tissue. Damage to tissue as a result of “tissue drag” friction causes inflammation and pain to the patient and leads to a longer recovery time. Friction may also damage the material, thus rendering it ineffective or shortening its useful life.
The reduction of tissue drag using biodegradable and non-biodegradable polymers as coatings on medical devices has been widely reported. Some of the biodegradable polymers reported as coatings to reduce the tissue drag of medical devices include polymers, copolymers, and blends containing monomers of lactide, glycolide, epsilpon-caprolactone, trimethylene carbonate, para-dioxanone, ethylene oxide, and propylene oxide.
Many implantable medical devices, such as hip or knee prostheses, are structured such that there is movement of a surface of the device against another surface of the device. This surface-to-surface movement may occur during either implantation, or during the life of the device. The force resulting from the relative movement, or articulation, of one surface against another, is known as “device drag”. In device drag, friction may damage the surface of the device, thus rendering it ineffective or shortening its useful life.
The issue of device drag in biodegradable and non-biodegradable implantable medical devices has been addressed in a variety of ways. Typically, low friction coefficient coating materials have been used to reduce device drag. Non-biodegradable coatings, such as ceramics or diamond like carbon, natural body fluids, or biodegradable polymers, have been reported as materials to coat on one or more of the contact surfaces to reduce friction between the surfaces.
Although considerable efforts have been applied to develop coating materials, there have been not many significant changes to the coating techniques for medical devices. The most often used coating techniques include spray coating, dip coating, wire coating, and powder coating. Although the mechanisms of these coating techniques are simple, the parameters involved are multiple.
For example, in spray coating, variables include the angle and the distance from the nozzle to the surface of the object, the opening of the nozzle, and the air pressure, all of which are key factors for the coating rate, which in turn determines the coating thickness. In addition, because of the uniqueness of medical device designs, custom-made device-specific gripping or holding systems are often required in the coating process. The use of the device-specific holding system requires the operators to periodically stop the coating process to remove coated objects and mount uncoated objects.
Another major aspect of the spray coating process is that organic solvents are widely used. The coating apparatus has to be placed in an environmentally safe location, which could make the cost of a simple coating process very expensive. In the case of medical devices formed from biodegradable polymers, a solvent must be chosen that does not adversely affect the polymer.
Finally, when a medical device is coated with a polymer, the performance of the coating is not only a function of the coefficient of friction of the coating material, but also of the interfacial bonding strength of the coating material and the device. Under high shear forces, the coating can delaminate from the device.
In summary, the problems of tissue drag and device drag in implantable medical devices have been of concern to the medical profession for some time. For medical devices formed from biodegradable polymers, both biodegradable and non-biodegradable coatings have been reported. However, the coating process is often complicated and involves solvents. Accordingly, there is a need for simpler methods of reducing tissue and device drag in biodegradable implantable medical devices.